*******Note that forecast and outlooks in this post are NOT the official forecast from the National Hurricane Center (NHC). They are my own detailed views on the Atlantic tropics based on current observations and latest computer model runs. As such do not make decisions based on my posts...consult news media...watches and warnings from your local weather office...and any evacuation orders issued by local governments to make the most informed and best decisions. Visit the NHC website hurricanes.gov (hurricanes dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.*********
...UPDATE...FRIDAY SEPTEMBER 27 2024 12:33 PM EDT...
The birdseye view chart valid for September 26 has now been added below. In addition... the following events have occurred since the full update below from 10:30 PM EDT last night:
(1) Helene's center made landfall at 11:20 PM EDT on the far eastern Florida panhandle coast... the stats were category 4 status with 140 mph maximum sustained winds and 938 mb minimum central surface pressure. Since then Helene has raced northward across the inland southeastern United States while steered around the east side of a developing cut-off upper vortex... with the worst wind and heavy rainfall flooding impacts felt across northern Florida (eastern Florida panhandle and northwestern parts of the peninsula)... Georgia... and the western Carolinas... however states surrounding this area are also seeing impacts from the inland circulation of Helene. I will have a special update in a couple of hours discussing more on the impacts from Helene.
(2) While continuing east in the open north Atlantic... Isaac intensified early and became a hurricane overnight. As of 11 AM EDT the stats are 75 mph maximum sustained winds with 981 mb minimum central surface pressure.
(3) The area of interest tagged as #32 in this post... in the open central tropical Atlantic... was upgraded to Tropical Storm Joyce at 11 AM EDT. The stats on the new storm are 40 mph maximum sustained winds with 1006 mb minimum central surface pressure.
...THURSDAY SEPTEMBER 26 2024 10:30 PM EDT...

This full update on the Atlantic tropics is released without the traditional birdseye view chart (satellite… surface… and upper air graphics) to ensure a timely release of this update given the urgent situation with rapidly intensifying Hurricane Helene. The chart will be added to this post later on.
The Atlantic tropics are rather active this evening with intense Hurricane Helene making landfall into the southeastern US from northern Florida… Tropical Storm Isaac roaming eastward in the open North Atlantic… and the following areas of interest:
(1) See area of interest #32 section below for the eastern Atlantic tropical wave now moving into the central Atlantic with a high chance of tropical cyclone formation
(2) See area of interest #34 for a tropical wave of low pressure which could develop in a few days while approaching Central America and the western Caribbean
(3) A tropical wave of low pressure with a distinct thunderstorm field is now in the far eastern tropical Atlantic over and south of the Republic of Cabo Verde Islands… however not currently adding it as an area of interest as models tend to agree that area of interest #32 will tend to dominate the surface inflow and upper outflow of the region.
(4) A currently disorganized Central Africa tropical wave of low pressure is expected to enter the tropical Atlantic in 3+ days. During this same time the current eastern and central tropical Atlantic upper ridge cells merge and push the current upper vorticity near the NW Africa coast southwest toward the Atlantic tropics. However models are increasingly suggesting this wave may develop due to a highly-divergent low-latitude upper ridge that becomes defined in the relatively higher pressures south of the upper vorticity… thus this wave may become the next area of interest for the eastern and central tropical Atlantic in the days ahead.
MAJOR HURRICANE HELENE... Satellite image of Helene reaching category 4 peak intensity just before landfall with north Florida… image taken at 0106Z:

Special update #89A from earlier this afternoon described the atmospheric setup leading to Helene’s recent bout of rapid intensification (https://www.infohurricanes.com/post/my-2024-atlantic-hurricane-season-birdseye-view-post-89a-special-update). We are in a worst-case scenario for coastal parts of the far eastern Florida panhandle and northwest Florida peninsula and areas just inland with catastrophic wind and coastal storm surge damage imminent… as the intensification process has led Helene to become a category 4 with 140 mph maximum sustained winds as of this update. After landfall... the south part of the steering upper trough is forecast to become a cut-off upper vortex to the east of an amplified upper ridge... with Helene arcing north then eventually west around the northeast half of the upper vortex... a path that takes the inland remnant system across the eastern US. During this time Helene transitions into a remnant frontal cyclone supported by the eastern divergence zone of the steering upper vortex.
Regarding impact to land areas:
(1) Coastal surf has arrived across the Florida west coast… Florida panhandle coast… and Alabama coast. The coastal surf is currently severe and life-threatening for the eastern Florida panhandle and northwestern Florida peninsula... preparations to protect your life from the surf/surge should have been completed by now.
(2) Given Helene's currently large wind field size... tropical storm gusts with isolated damage potential are now occurring across all of the Florida peninsula... as well as the eastern and central part of the Florida panhandle. Severe wind damage from stronger tropical storm to major hurricane force winds are imminent across the northwestern Florida peninsula... eastern panhandle... and southern Georgia overnight. As a large remnant frontal cyclone... ex-Helene could bring a large swath of gusty winds with scattered damage potential across Georgia... Alabama... Mississippi... eastern Arkansas... southeastern Missouri... Tennessee... Kentucky... southern Illinois... southern Indiana... southern Ohio... the western Virginias (including most of West Virginia)... and the western Carolinas through Friday and Saturday.
(3) Expect heavy rainfall with flash flooding potential across all of Florida... Georgia... Alabama... Mississippi... eastern Arkansas... southeastern Missouri... Tennessee... Kentucky... southern Illinois... southern Indiana... southern Ohio... the Virginias... and the Carolinas through Friday and Saturday.
(4) Tornadoes are possible across the northern Florida peninsula… eastern Florida panhandle… Georgia… and the Carolinas over the next several hours as directional wind shear favoring severe thunderstorms and tornadoes is present (combo of southeasterly surface flow in advance of Helene and upper southwesterlies ahead of the eastern North America upper trough). Interests here should listen to local news media to be prepared to shelter from tornadoes.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0000Z Sep 27)... 140 mph maximum sustained wind major hurricane centered just offshore of the far eastern Florida panhandle at 29.1N-84.2W
IOH 12 Hr Forecast (1200Z Sep 27)... Remnant frontal cyclone centered over the northwestern corner of South Carolina at 34.5N-82.8W
*****National Hurricane Center (hurricanes.gov) official forecast as of 5 PM EDT*************************
60 Hr Position (0600Z Sep 29)... 25 mph maximum sustained wind dissipating remnant low centered over southwestern Kentucky at 37.1N-87.3W
AREA OF INTEREST #32... The organized tropical wave of low pressure in the eastern tropical Atlantic will soon be crossing into the central tropical Atlantic. Its central area of rotating thunderstorms has been pulsing up and down in intensity (on colorized infrared satellite pictures) instead of remaining steadily strong... an indication the band of nearby dry Saharan air to the north continues to affect this system. My track forecast is the same as previous... with a westward adjustment to the long-range north then northeast curving track. This is in alignment with the latest model consensus showing the deep-layer ridge weakness... which this system is expected to turn north into... being narrower and further west. The deep-layer ridge weakness will be a combination of tropical cyclone Isaac and overhead upper vorticity merging with the current eastern Canada frontal low and its upper trough. My intensity forecast is lowered given that tropical cyclone formation still has not occurred as of this writing... calling for gradual intensification through 72 hours despite the presence of the current overhead upper ridge with low shear and upper outflow... due to the current dry Saharan air environment. After that time upper southwesterly shearing flow associated with the deep-layer ridge weakness begins to negatively affect this system... and I call for weakening from 72 to 120 hours. The weakening rate is shown to be gradual as the field of upper southwesterly flow will also be divergent which may help this system fire thunderstorm bursts while sheared.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 26)... Tropical low centered in the eastern tropical Atlantic at 15.5N-37.5W
IOH 24 Hr Forecast (1800Z Sep 27)... 40 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 16N-41.2W
IOH 48 Hr Forecast (1800Z Sep 28)... 60 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 18.5N-44W
IOH 72 Hr Forecast (1800Z Sep 29)... 70 mph maximum sustained wind tropical storm centered in the central Atlantic at 21N-47.5W
IOH 96 Hr Forecast (1800Z Sep 30)... 60 mph maximum sustained wind tropical storm centered in the central Atlantic at 24N-46W
IOH 120 Hr Forecast (1800Z Oct 1)... 50 mph maximum sustained wind tropical storm centered in the central Atlantic at 27.5N-44W
*****National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT*************************
Formation chance through 48 hours... 90%
Formation chance through 7 days... 90%
TROPICAL STORM ISAAC… The tropical storm in the open North Atlantic and the overhead upper vorticity are continuing east in tandem… and will continue to do so through 24 hours. After that time the upper vorticity merges with the current eastern North America upper trough… with the merged cold core upper trough axis being west of Isaac as the upper trough fragment from North America is colder than the overhead upper vorticity. This system’s track therefore bends more north after 24 hours in the southwesterly flow ahead of the merged upper trough axis. There is some discrepancy in the models for the 4+ day track forecast… with the CMC… ECMWF… and NAVGEM favoring this system turning less north and more east toward Europe while caught by the southwest side of a high-latitude upper trough that dives in from northern Canada and Greenland… and the GFS favoring the merged upper trough axis to the west amplifying into an upper vortex that hooks this system north. For now my day 4 and 5 positions splits the difference between these two extremes. For intensity… Isaac appears to be at equilibrium… and conditions do not change over the next 24 hours so I show no strengthening through that time. By 48 and 72 hours… despite moving into waters below 26 deg C… an immense amount of upper divergence on the east side of the merged upper trough axis may help Isaac make a run at hurricane strength. By 96 hours… my forecast position keeps Isaac under some upper divergence… however with Isaac nearing cooler 20 deg C water I lower the intensity forecast. And for 120 hours my forecast track curves Isaac east in the flow across a mid-latitude upper ridge wave that will be southwest of the aforementioned high-latitude upper trough. However while remaining at 20 deg C waters… coupled with the warm stabilizing temps of the upper ridge wave… the thermodynamic profile is unfavorable for Isaac to take advantage of the upper ridge wave’s outflow as a tropical system. Thus by day 5 I forecast Isaac to continue weakening while being downgraded to a non-tropical surface low lacking thunderstorms.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 26)... 50 mph maximum sustained wind tropical storm centered in the central Atlantic at 37.3N-51.7W
IOH 24 Hr Forecast (1800Z Sep 27)... 50 mph maximum sustained wind tropical storm centered in the central Atlantic at 37.5N-45W
IOH 48 Hr Forecast (1800Z Sep 28)... 65 mph maximum sustained wind tropical storm centered in the northern Atlantic at 40N-40W
IOH 72 Hr Forecast (1800Z Sep 29)... 75 mph maximum sustained wind hurricane centered in the northern Atlantic at 43N-35W
IOH 96 Hr Forecast (1800Z Sep 30)... 60 mph maximum sustained wind tropical storm centered in the northeastern Atlantic well north of the Azores at 46N-30W
IOH 120 Hr Forecast (1800Z Oct 1)... 50 mph maximum sustained wind surface low centered in the northeastern Atlantic at 47N-22W
*****National Hurricane Center (hurricanes.gov) official forecast as of 5 PM EDT*************************
Peak Strength (0600Z Sep 28)… 75 mph maximum sustained wind hurricane centered at 39.2N-42W
5-Day Position (1800Z Oct 1)... 45 mph maximum sustained wind surface low centered in the northeastern Atlantic at 45.5N-22.5W
AREA OF INTEREST #34… A tropical wave of low pressure is now crossing the Lesser Antilles and heading into the eastern Caribbean while producing thunderstorm squalls across the central and northern Lesser Antilles. However over the next couple of days… as the surface wave crosses the eastern and central Caribbean… it is likely to be suppressed as (1) the warm core upper ridge over and east of Helene eventually matures into a west Atlantic deep-layer ridge… (2) the developing deep-layer ridge is already spreading upper vorticity currently to the west of the wave across the southern half of the Caribbean… (3) the deep-layer ridge pushes some of the upper vorticity over Isaac southwest into the southeast Caribbean by day four… (4) the tropical wave becomes suppressed by a lack of divergence beneath the upper vorticity. Noting on days 3 and 4 the upper air pattern over the wave begins to improve as the upper vorticity in step (2) above fades and allows the southwest side of the deep-layer ridge to potentially increase the outflow over the wave… however I assign a low 10% chance of development as any models that do develop this surface wave prefer to show development after that time. Noting that long-term development after day 4 may be briefly interrupted by the upper vorticity in step (3) above sliding west over the surface wave. Thus I assign a 0% chance of development on day 5… however just after that time this second wave of upper vorticity is then expected to collapse from its isolation from high-latitude cold air which allows for an expanding upper anticyclone (low shear/outflow) to form in its wake and over the surface wave. Therefore the long term prospects of the surface wave’s development currently look good.
Noting the forecast track for this area of interest is initially west around the aforementioned west Atlantic deep-layer ridge. Then by day 5 the track begins to bend northwest around the southwest edge of the ridge and due to its interaction with the east side of the collapsing second wave of upper vorticity.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
Formation chance through 4 days... 10% (Caribbean Sea just east of the Nicaragua/Honduras border near 15N-82W)
Formation chance days 4 to 5… 0% (offshore of eastern Honduras near 16N-84W)
*****National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT*************************
Formation chance through 48 hours... 0%
Formation chance through 7 days... 20%
...COMPUTER MODEL SUMMARY...
Source...Tropical Tidbits (https://www.tropicaltidbits.com/analysis/models/)
0000Z (Sep 26) CMC Model Run...
**For Hurricane Helene... center makes landfall over the eastern Florida panhandle between 24 and 30 hours... while transitioning into a large and vigorous inland remnant frontal cyclone the center whirls north-northwest into western Kentucky/Tennessee border at 54 hours while reaching the core of the regional overhead upper vorticity... while weakening into a large remnant frontal low due to a lack of divergence beneath the overhead upper vorticity ex-Helene dissipates at 90 hours while drifting east into the central Kentucky/Tennessee border
**For Tropical Storm Isaac... while continuing east-northeast transitions into a more elongated non-tropical frontal low while located north-northeast of the Azores (43.5N-22.5W) at 96 hours... remnant low then proceeds to dissipate while drifting east-southeast toward the coast of Portugal through 144 hours
**For area of interest #32... tropical cyclone formation suggested near 17.5N-45W at 48 hours... while reaching hurricane strength turns north then northeast and reaches 36N-41W through 168 hours
**For area of interest #34... tail end of surface cold front driven by ex-Helene reaches waters just west of the Yucatan peninsula through 78 hours while simultaneously the tropical wave currently just east of the Lesser Antilles proceeds to move into and across the Caribbean... tail end of front and tropical wave then proceed to merge into a broad tropical low over Central America... southeastern Mexico... and adjacent Caribbean and eastern Bay of Campeche waters through 102 hours... lowest pressure center of the broad tropical low pressure field proceeds to move west from the western Caribbean and into southeastern Mexico through 168 hours
**Tropical wave currently just offshore of western Africa develops into a tropical low that passes over the Republic of Cabo Verde Islands through 24 hours... subsequently weakens back to a wave while located near 16N-35.5W at 66 hours with no re-development shown afterwards
**Additional tropical wave emerges from west coast of Africa at 60 hours and proceeds to gradually evolve into a tropical low that reaches the central tropical Atlantic near 14.5N-42.5W by 168 hours
0000Z (Sep 26) ECMWF Model Run...
**For Hurricane Helene... center makes landfall over the eastern Florida panhandle at 27 hours... while transitioning into a large and vigorous inland remnant frontal cyclone the center whirls north-northwest into southern Illinois through 57 hours while reaching the core of the regional overhead upper vorticity... while weakening into a large remnant frontal low due to a lack of divergence beneath the overhead upper vorticity ex-Helene dissipates at 108 hours while quasi-stationary over the IL/ IN/ KY border intersection
**For Tropical Storm Isaac... continues east-northeast and passes north of the Azores through 105 hours... while continuing to maintain a compact circular surface low pressure field reminiscent of a system with tropical characteristics proceeds to move east-southeast into the coast of Portugal by 150 hours... while an inland decaying remnant low Isaac moves into southwestern Spain through 168 hours
**For area of interest #32... continues west-northwest to 18N-49W through 99 hours as a tropical low without tropical cyclone formation... while becoming quasi-stationary at this region dissipates at 135 hours
**For area of interest #34... tail end of surface cold front driven by ex-Helene reaches waters just west of the Yucatan peninsula through 72 hours while simultaneously the tropical wave currently just east of the Lesser Antilles proceeds to move into and across the Caribbean... tail end of front and tropical wave then proceed to merge into a broad tropical low over Central America... southeastern Mexico... and adjacent Caribbean and eastern Bay of Campeche waters through 117 hours... no development shown within the quasi-stationary broad tropical low through 168 hours
**Tropical wave currently just offshore of western Africa develops into a tropical low that passes over the Republic of Cabo Verde Islands through 21 hours... subsequently weakens back to a wave while located near 17N-39W at 87 hours with no re-development shown afterwards
**Additional tropical wave emerges from west coast of Africa at 66 hours and proceeds to gradually evolve into a tropical low that reaches the central tropical Atlantic near 13N-46W by 168 hours
1200Z (Sep 26) GFS Model Run...
**For Hurricane Helene... center makes landfall over the eastern Florida panhandle between 12 and 18 hours... while transitioning into a large and vigorous inland remnant frontal cyclone the center whirls north-northwest into the Illinois/Kentucky border through 42 hours while reaching the core of the regional overhead upper vorticity... while weakening into a large remnant frontal low due to a lack of divergence beneath the overhead upper vorticity ex-Helene dissipates at 78 hours while drifting east into central Kentucky
**For Tropical Storm Isaac... continues east-northeast to 45N-32W through 102 hours... the frontal low/upper trough currently over eastern Canada then proceeds to become a deep-layer cyclone just west of Isaac which causes Isaac to turn 51N-33W through 132 hours while it transitions into a remnant frontal cyclone supported by the upper divergence zone of the deep-layer cyclone... remnant frontal cyclone then turns east and rapidly weakens to a frontal low while caught by suppressive southwestern upper convergence zone of high-latitude upper trough... reaching 51N-25W by 168 hours where it dissipates soon after
**For area of interest #32... compact tropical cyclone formation suggested near 17N-42.5W at 24 hours... reaches peak strength as a compact tropical storm near 19.5N-46.5W at 54 hours... while turning north weakens to a remnant low near 24.5N-46W by 120 hours... remnant low in the open central Atlantic loses identity at 150 hours while merging with cold front driven by the ex-Isaac and northwestern Atlantic frontal cyclone pair
**For area of interest #34... tail end of surface cold front driven by ex-Helene reaches waters just west of the Yucatan peninsula through 72 hours while simultaneously the tropical wave currently just east of the Lesser Antilles proceeds to move into and across the Caribbean... tail end of front and tropical wave then proceed to merge into a broad tropical low over Central America... southeastern Mexico... and adjacent Caribbean and eastern Bay of Campeche waters through 108 hours... east end of broad tropical low develops a well-defined center near 17.8N-82W at 144 hours shortly after which time it becomes a tropical cyclone... the tropical cyclone proceeds northwest to 20.5N-85.5W (northwestern Caribbean) through 168 hours
**Tropical wave currently just offshore of western Africa develops into a tropical low that passes over the Republic of Cabo Verde Islands through 12 hours... subsequently weakens back to a wave while located near 15N-37W at 90 hours with no re-development shown afterwards
**Additional tropical wave emerges from west coast of Africa at 48 hours and proceeds to gradually evolve into a tropical cyclone near 10.5N-32W by 168 hours
0600Z (Sep 26) NAVGEM Model Run...
**For Hurricane Helene... center makes landfall over the eastern Florida panhandle at 24 hours... while transitioning into a large and vigorous inland remnant frontal cyclone the center whirls north-northwest into far western corner of Kentucky through 54 hours... while quasi-stationary at this location through 96 hours ex-Helene proceeds to weaken to a large remnant frontal low due to a lack of divergence beneath the overhead upper vorticity core... remnant frontal low then proceeds to accelerate northeast into southeastern Michigan through 126 hours where it then loses identity to much larger and stronger frontal cyclone approaching from central Canada
**For Tropical Storm Isaac... continues east-northeast and passes well north of the Azores by 60 hours... while energized by the eastern divergence zone of southeastward-diving high-latitude upper trough proceeds to strengthen into a large and powerful remnant non-tropical frontal cyclone that moves into the northern British Isles by 102 hours
**For area of interest #32... tropical low becomes quasi-stationary near 18N-51W from 114 to 144 hours
**For area of interest #34... no development shown through 144 hours
**Tropical wave currently just offshore of western Africa passes over the southern Republic of Cabo Verde Islands as a tropical low through 18 hours... subsequently absorbed by east side of area of interest #32 at 114+ hours
**Additional tropical wave emerges from west coast of Africa at 60 hours and proceeds to gradually evolve into a tropical low that reaches 12N-30W by 168 hours
Opmerkingen